1. Technical Field
The present invention relates to an optical reception device and an optical reception method, both for performing a digital coherent reception process, and in particular, relates to an optical reception device having a configuration wherein an optical front-end unit formed into a pluggable module is attached, after the formation, to a digital signal processing unit, and also relates to an optical reception method using the device.
2. Background Art
In recent optical transmission systems, the amount of data transmitted through an optical transmission channel has been steadily increasing. In view of enabling to accommodate such data amount, there is strong demand for capacity increase also of optical transmission channels. As one of technologies capable of realizing such capacity increase, there is a technology of increasing the frequency utilization efficiency during wavelength multiplexed transmission by means of multi-level modulation.
In an optical transmission system of a high order multi-level modulation method, it is usual to employ a coherent detection wherein a received phase-modulated signal is detected by combining it with local oscillator light. In that case, in order to compensate for a difference between the carrier frequency of the optical signal and the frequency of the local oscillator light and for fluctuation in their phases, carrier phase estimation and compensation are performed, using digital signal processing technology, at an optical front-end unit and a digital signal processing unit. Accordingly, in an optical reception device adopted in a digital coherent optical transmission system, it has been general that an optical front-end unit and a digital signal processing unit are formed together into a single body.
A configuration being employed in recent years is that obtained by forming an optical front-end unit into a pluggable module and attaching it, after the formation, to a digital signal processing unit. However, when the two units are connected with each other by means of a transmission wire, a high frequency component of a signal is attenuated during the signal's propagating through the transmission wire, because general transmission wires function as a low pass filter.
In FIG. 6, shown is an input signal characteristic (a) at a digital signal processing unit in a case where the digital signal processing unit and an optical front-end unit are formed together into a single body. In FIG. 6, also shown is an input signal characteristic (b) at a digital signal processing unit in a case where the optical front-end unit is formed into a pluggable module and then is connected with a digital signal processing unit by means of connectors and a transmission wire of 50 mm length. Here, in the former optical reception device, the optical front-end unit and the digital signal processing unit are connected with each other by means of a transmission wire of 10 mm length. As seen from FIG. 6, a high frequency component is remarkably attenuated in the input signal characteristic (b) for when the front-end unit and the digital signal processing unit are connected with each other by means of connectors and a transmission wire, compared to the input signal characteristic (a) for when the two units are formed together into a single body. The attenuation of a high frequency component increases with increasing frequency.
As technologies for compensating for attenuation of a high frequency component, there are a technology which amplifies a high frequency component in advance at the sending side and thereby cancels out deterioration of a high-frequency signal to occur before its arrival at the receiving side (pre-emphasis), and a technology which performs, according to the characteristic of an already generated signal, feedback control of the frequency characteristic of a signal to be newly generated. For example, an optical transmission system employing pre-emphasis is disclosed in Japanese Patent Application Laid-Open No. 1997-261205 (Patent Literature 1), and an optical reception device performing feedback control of the frequency characteristic of a signal to be newly generated is disclosed in Japanese Patent Application Laid-Open No. 1994-104670 (Patent Literature 2).